Electromagnetic actuators contain one or more cylindrical coils of insulated wire called solenoids (hereinafter referred to as coils). Energization of a coil by a dc voltage source creates a steady current flow through the coil which produces an axial magnetic field. The direction of the magnetic field is dependent upon the direction in which the coil conductor is wound and the direction of the current flow through the coil.
Electromagnetic actuators also contain a movable ferromagnetic component called an armature. The armature is located within the magnetic field and is subjected to a force, created by the field, which is proportional to the strength of the field. This force causes the armature to move in the same direction as the magnetic field.
Deenergization of a coil (removing the dc voltage source) interrupts the current flow. As a result, the magnetic field collapses and the force dissipates. Typically, the armature is returned to its original position by means of a spring.
The time it takes an armature to complete its movement in the direction of the magnetic field upon energization of the coil, and the time it takes the armature to return to its original position upon deenergization of the coil, are hereinafter referred to as the armature response times.
Armature response times are affected by several factors, including the number of coil turns, the density of the coil windings (turns per meter), the intrinsic resistance of the coil conductor, the distance between the armature and the coil, the metallurgical composition of the armature, return spring forces and the internal clearances around movable elements. These factors, among others, will vary among identically constructed actuators as a result of manufacturing variations within allowable design tolerances and are significant enough to result in a range of response times. When required to work in concert with one another to exacting specifications (e.g., electromagnetic fuel injectors in a multiple cylinder internal combustion engine), or when replacing an actuator with one manufactured to the same "specification," the above-noted variations may become unacceptable.
One approach is to tighten the allowable tolerances. Unfortunately, the manufacture of any device is limited by the accuracy and consistency of the equipment used and their respective operators. Often, identical devices are produced by different manufacturers with different equipment. Even when produced by the same manufacturer, equipment functionality and operator performance will vary from day to day. Therefore, metallurgical and dimensional differences between actuators manufactured to identical specifications are inevitable, so allowable tolerances can be tightened only so much.
Another approach to the problem would be to provide an internal means of adjusting key components of the actuator itself (e.g., screw to adjust spring force, post-manufacture modification of core to adjust permeability, etc.). Such methods complicate the manufacturing process, inflate the actuator cost and necessitate a time-consuming calibration process typically requiring partial disassembly of the actuator, thereby subjecting it to contamination.